The present invention relates to improvements in steam generating boilers. More particularly, the invention is directed toward improving combustion conditions in steam generating boilers such as the recovery boilers which are used in the pulp and paper mills for the combustion of spent liquor from sodium-based pulping process.
The main objectives in operating a recovery boiler are to recover the pulping chemicals in the reduced state and to recover the heat released by the combustion of carbonaceous material to generate steam for the process. The spent liquor from the pulping process is sprayed in small drops over the cross-section of the boiler furnace through the upwardly flowing combustion gases so as to dry the liquor droplets to a concentration where the heat value. of the char material with the residual moisture is sufficient to keep a reasonably stable combustion going. The dry liquor solids settle on the bottom of the boiler forming a carbonaceous char bed. The char bed has two functions: to reduce the spent chemicals for further recycle and to supply heat by reacting with the oxygen in the air being blown horizontally over the char bed.
In such boilers, the air is introduced peripherally through ports located in the boiler sidewalls and into the lower section of the boiler. In most designs, the total air supply is divided in two or more streams which are introduced peripherally at different levels of the boiler. These air streams are referred to as primary, secondary and tertiary air, conventionally starting from the bottom of the boiler.
Because of the influence of the induced draft fan and of the large size of the boiler, only a very small fraction of the peripherally introduced air reaches the central region of the boiler's cross-section.
Peripheral air is introduced either horizontally or slightly downwardly at subsonic velocities ranging from about 25 to 100 m/sec, which causes an upward deflection of the air along the walls of the boiler.
In cases where the fuel which supplies heat for the steam generation is concentrated towards the center of the boiler's cross-section, such as in the case where a char bed containing carbonaceous materials sits on the bottom of the boiler, the peripherally introduced air will not readily combine with the combustible species, either gaseous or finely divided solids, resulting in poor combustion in the lower section of the boiler.
A fundamental limitation to the burning capacity of these boilers is due to such poor mixing between the combustible species and the oxygen required as a comburant. As the air is introduced peripherally through sidewall ports and blown into the lower section of the boiler, due to the relative low pressure and subsequent low velocity of the air flow, there is a preferential upward flow along the sidewalls, leading to poor mixing with the combustible species.
The lack of intimate mixing of air with the combustible species in the lower section of the boiler limits its capacity not only because heat transfer to the boiler tubes is poor since peripheral air behaves as a coolant, but also because the lack of mixing lengthens the combustion zone, resulting in a vertical temperature profile which promotes carry over of unreacted inorganic material and in unnecessarily higher temperatures in the upper section of the boiler, where screen tubes and superheater tubes are located.
Layers of carried over deposits on the screen and superheated tubes can be over 20 mm thick, thus drastically obstructing heat transfer and reducing the sectional area for the passage of gases, eventually bottlenecking the boiler when the high pressure drop through the upper section limits the air blowing capacity of the boiler, forcing scheduled or non-scheduled shut-downs for deposit removal.
Another problem associated with poor mixing of the air with the combustible species is the emission of reduced sulfur species in the exit gas. In conventional boilers, eventhough an overall O.sub.2 excess of over 2% may exist in the exit gas, some reduced sulfur species mix with the available oxygen only at the top of the boiler, where not enough time is available for a complete oxidation to occur and/or the gases are already at a lower temperature than necessary for complete oxidation.
From a thermochemical equilibrium view point, as long as there is more than 1% vol. O.sub.2 in the flue gases exiting the boiler furnace, there should be less than 20 ppm total reduced sulfur (TRS) species. However, because of imperfect gas mixing, equilibrium is not attained and therefore oxygen and combustible species coexist in the flue gases.
A better mixing of oxygen and the combustible species would modify the vertical temperature profile of the boiler resulting in a temperature increase in the lower section of the boiler and consequent shorter combustion zone and lower exit gas temperature in the upper section of the boiler with the following advantages:
1. Reduction of carried over deposits. PA1 2. Lowering of TRS emissions at similar excess O.sub.2 in flue gas. PA1 3. Increased chemical recovery capacity. PA1 4. Increased steam generating capacity. PA1 5. Reduced shut-down frequency for deposit removal. PA1 6. Smoother boiler operation.
Enriching the combustion air with oxygen would further allow burning capacity increases without subsequent increase in carry over deposits due to lower gas velocities relative to air combustion.